1. Field of the Invention
The present invention relates to a piezoelectric oscillator having a tuning fork piezoelectric vibrating piece sealed in a package and a method of the same, and to a technique in which a thin film attached on a piezoelectric oscillator is removed to adjust oscillation frequencies.
2. Description of the Related Art
As a method of adjusting an oscillation frequency in the case of fabricating a piezoelectric oscillator such as a tuning fork crystal oscillator, there is a method in which a trimming area is provided on a device and a thin film attached as a weight material is trimmed by a laser beam. As this type of technique, for example, a method of fabricating a piezoelectric oscillator and a piezoelectric device described in Patent Reference 1 is known (see JP-A-2003-133879). In the piezoelectric oscillator described in Patent Reference 1, as shown in FIG. 9, on an oscillating arm 1A of a tuning fork crystal vibrating piece 1, a weight portion 3 is provided, having a mass adjustment film 2 that the mass is increased and decreased to adjust oscillation frequencies. The mass adjustment film 2 is formed of a deposition film 4 (coarse tuning portion) that Au is deposited to be a weight for coarse tuning, and a sputter film 5 (fine tuning portion) that Cr is sputtered to be a weight for fine tuning, in which on the top surface of the tuning fork crystal vibrating piece 1 below the weight portion 3, a predetermined electrode 6 is patterned by the photolithographic technique. Then, in the piezoelectric oscillator, as shown in FIG. 10, the deposition film 4 and the sputter film 5 of the weight portion 3 are trimmed with a laser beam to partially form a plurality of film removal spots having the diameter matched with the spot diameter of laser for adjustment so that spot trains 7 and 8 formed of a plurality of the film removal spots allow the frequency of the tuning fork crystal vibrating piece 1 to be a target frequency.
From the recent trend in miniaturization of components, oscillators are also shrunk in size, and the frequency change per unit mass of the weight used for trimming becomes greater. In order to adjust frequencies highly precisely, it is necessary to finely control the trimming amount of the weight. However, it is difficult to provide a smaller spot diameter of the laser beam for trimming, which becomes a problem of securing precise oscillation of a small-sized oscillator. In order to cope with this problem, the following schemes (1) and (2) are shown.
(1) Such laser is used that the spot diameter of laser can be made smaller. When an amount of processing is still necessary, the deposition film 4 to be the weight for coarse tuning and the sputter film 5 to be the weight for fine tuning are trimmed, in which as shown in FIG. 11, the sputter film 5 is first trimmed with a larger spot diameter (indicated by a sign 8A), and is trimmed with a smaller spot in fine tuning that requires accuracy (indicated by a sign 8B).
(2) As shown in FIG. 12, the pitch of the later spot is reduced to “P2” from “P1” which is the diameter of the spot in order to decrease the trimming amount per unit pulse (see Patent Reference 1). In order to secure adjustment accuracy, trimming is conducted in the lateral direction in which an amount of change per unit mass of the weight becomes constant. When trimming of a train is finished, the diameter of the later spot is set to a fixed amount (d1), which is the same as the spot diameter, and the later spot is moved in the column direction (the vertical direction in this drawing), and then the subsequent train is processed.
In the trimming scheme conducted by changing the spot position of laser, the following problems arise. In the scheme (1), although accuracy can improve while securing the adjustment efficiency, in a standard laser trimming apparatus, the diameter is generally about 20 μm at most even though the spot diameter is made smaller. A laser trimming apparatus having the spot diameter smaller than that diameter is very expensive, which increase production cost. In addition, in the scheme (2), the resolution of adjusting frequencies in the lateral direction can be improved when the pitch can be set small as indicated by “P1>P2”. However, as the oscillator reduced in size has the small width of the tuning fork crystal vibrating piece 1 and the small weight portion 3 at the end part thereof. Accordingly, it is unable to provide a long trimming length. For this reason, the application position is to be shifted in the column direction. Generally, because a feed d1 to be set is about the spot diameter of the laser, it is unable to implement frequency adjustment with high precision according to trimming based on this feed.